1. Field of the Invention
The present invention relates to an optical instrument and optical method for measuring displacement of a scale. The present invention particularly relates to the optical instrument and method for measuring a position and/or velocity of a mobile body.
2. Description of Background Art
Japanese Patent Application First Publications No. Showa 47-10034 (United States Patent Application Ser. No. 73, 791 filed on Sep. 21, 1970) published on May 22, 1972, No. Showa 63-75518 published on Apr. 5, 1988, and No. Heisei 2-85715 published on Mar. 27, 1990 exemplify previously proposed optical displacement measuring apparatuses which detect positional changes of moving diffraction gratings utilizing interferences in light.
In each of the previously proposed optical displacement measuring apparatuses, a light flux (beams) radiated from a coherent light source is split into two light fluxes by means of a beam splitter, the two light fluxes are advanced through respectively separate light paths and incident on a diffraction grating constituting a scale, and generated diffraction lights thereon in an m order (m denotes 1 or larger integer) are obtained, and these two m-order diffracted light are recombined and a diffraction fringe of the recombined light flux is detected.
A light source and optical detector are of transmission types in which they are on opposite sides of the scale and of reflection types in which they are in the same positions with respect to the scale.
In general, a volume type hologram is used as the diffraction grating for the transmitting type application. Especially, if the optical parts are arranged such as to satisfy a bragg condition, a signal having a high output and high resolution can be derived.
However, since the transmitting type requires the arrangements of the light source and detector such that they are disposed in mutually opposite positions to a scale, the compact optical displacement apparatus cannot be achieved.
On the contrary, since the reflecting type requires the arrangements of light source and detector on the same positions to the scale, it is suitable for compacting the optical displacement instrument but it is difficult to achieve a high resolution instrument.
With such a tradeoff as described above in mind, an optical displacement measuring instrument having the advantages of both the transmitting type with those of the reflecting type may be considered. However, conventional problem attempts incurring in a that the adjustments of positions of the optical parts become very difficult.
FIG. 1 shows another previously proposed optical displacement instrument (encoder) disclosed in a Japanese Patent Application First Publication No. Heisei 4-130220 published on May 1, 1992.
In FIG. 1, the light beam (light flux) radiated From the light source 901 is incident on a first diffraction grating 902 passing through a corimeter lens 911, a beam splitter 903, and .lambda./4 plate 912. At this first diffraction grating, 902 the light beam is split into the two beams, receiving the diffraction at a second diffraction grating 906 forming the scale surface. The diffracted lights are incident vertically on a reflected plane 907. The reflected lights therefrom return to the original light path and are again made incident on the second diffraction grating 906.
The diffracted light diffracted by the second diffraction grating 906 is further returned to the original light path and again incident in the first diffraction grating 902. The combined light flux is returned to the beam splitter 903 and deflected toward an photo detector 904.
As described above with reference to FIG. 1, the light fluxes present between the diffraction grating 906 and reflecting plane 907 are vertical on the reflecting plane 907, i.e., the light fluxes are reciprocated in tile vertical direction as viewed from FIG. 1. Thus, the alignment adjustment between a reading optical system first diffraction grating 902 and the scale side second diffraction grating 906 is easy.